1. Field of the Invention
The present invention relates to a gastight piston ring arrangement including a piston ring made up of a plurality of piston ring segments, the ends of which are shaped such that they are axially stepped in the peripheral direction, and the stepped ends of adjacent piston ring segments, for the formation of an axial seal, are arranged lying axially one against the other.
2. The Prior Art
In piston compressors, the working fluid is compressed by a piston. The sealing elements which seal the annular gap formed between the piston and the cylinder are called piston rings. In piston compressors in double-acting construction, the gas pressure in the cover-side and crank-side compression chamber respectively fluctuates between suction and discharge pressure, the two working chamber pressure patterns being mutually out of phase by a 180° crank angle. In such applications, the piston sealing system must consequently seal in both directions, i.e. must be of double-acting configuration. In high pressure applications, on the other hand, the compression is often effected solely on the cover side, so that the piston rings in such applications always seal in one direction and reduce the varying compression chamber pressure to a normally smaller, time invariant pressure, for example the suction pressure of the stage in question. For such applications, single-acting piston rings should be used, thereby ensuring that, during the re-expansion phase, no unnecessarily high pressure is trapped in the radial gap volume between two piston rings, which would entail an unnecessary increase in piston ring wear.
Regardless of the degree of complexity of the construction, all piston rings share the feature that they are activated by the pressure differential to be sealed, i.e., are pressed with a certain surface pressure against the groove flank and the liner of the cylinder and thereby develop their sealing effect. Especially in dry-running applications, the ring wear rate, as a result of excessive surface pressures, can take on unacceptable proportions. The wear rate of PTFE-based materials, which are typically used in piston rings, generally grows considerably as the temperature of the opposite-working surface (in this case the liner) increases. The friction heat generated in the piston ring/liner sealing gap results in heating-up of the liner and, therefore, plays a considerable part. More specifically, if the rings are exposed to elevated temperatures, extrusion of the piston rings into the annular gap which is formed between the piston and the cylinder and which, in order to prevent the piston from rubbing against the liner, must have a certain minimum radial measure, can additionally markedly reduce the sealing effect of the piston rings or even destroy the piston rings. In dry-running applications, the concept of pressure balancing, where the pressure decay in the piston ring/liner sealing gap is influenced by grooves, slots, bores in the piston ring or the like such that the mean, surface-pressure-reducing gas pressure in this sealing gap is increased, is therefore made use of, especially in high pressure applications.
The way in which, in a plurality of mutually adjoining piston rings, the total pressure differential to be sealed is distributed to the individual piston rings depends on the individual relations between the prevailing pressure differential and leakage of the individual piston rings. By far the greatest contribution to the leakage by a piston ring is usually made by the gas flow through the joint of the piston ring. Since this joint grows bigger with increasing wear, the sealing effect of the rings also decreases markedly over time. The simplest possible construction of such a piston ring is here constituted by the one-piece, straight-cut form. One-piece, single-cut piston rings can only be fitted in composite pistons or can be fitted in one-piece pistons only by additional effort (for example, through the use of an additional tool or with previous warming), insofar as the piston rings are sufficiently flexible. Especially in piston rings with small diameters and thus high bending rigidity, as are used, in particular, in high pressure applications, only composite pistons can therefore be used, which is more laborious in terms of production and maintenance.
The tendency toward increasing leakage can be countered by the use of so-called gastight piston ring constructions in which the joint (in a single-cut ring) or joints (in a multi-cut segmented ring) is/are covered. A respectively cut L-shaped and right-angled ring are in this case combined, the joints of the two rings being arranged mutually offset. Such piston ring constructions are known, for example, from DE 28 29 352 A1 or from U.S. Pat. No. 3,632,121 A. Particularly in high pressure applications, where such ring constructions, due to greater seal-tightness requirements, are increasingly used, it often occurs, however, that a ring, due to the thermal and mechanical load, is forced into the joint of the other ring, which can lead to deformations and a deterioration in the sealing effect. In addition, fractures can occur as a consequence of the material weakening resulting from fully (i.e. in the radial and axial directions) covered joints.
Similarly, gastight piston ring constructions with segmented piston rings have already been known, in which a number of ring segments are combined to form a piston ring. The fundamental advantage of such segmented piston rings lies in the fact that such rings, as a result of their multipiece nature, can also easily be used in one-piece, non-composite pistons. Such a piston ring is described, for example, in U.S. Pat. No. 2,055,153 A. The sealing in the radial and axial directions is here effected by the shaping of the ring segments. Due to the necessary sealing in the radial and axial directions, a complex shape of the ring segments is obtained, however, which is complicated and difficult to produce. Such a piston ring is therefore laborious in terms of its production.
In another version of a gastight piston ring arrangement according to JP 2001-032935 A2, a single-cut piston ring having a stepped joint for axial sealing is combined with a single-cut sealing ring which bears against the radially inner peripheral surface of the piston ring and forms a seal in the radial direction. On the piston ring a projection is further provided, which engages in the joint of the sealing ring and thus prevents twisting and possible alignment of the piston ring and sealing ring joints. The sealing ring must, however, be readjustable in the event of increasing wear, i.e. it must be capable of expansion in order to maintain the seal in the radial direction. For a sufficient sealing effect, moreover, the sealing ring must be pressed against the piston ring with a certain minimum contact pressure. These requirements call for a sufficiently pliable sealing ring. The sealing ring would therefore need to be realized with small cross section. The effect of this, however, is that the sealing ring, especially in high pressure applications, can deform into the joint gaps of the piston ring, which would in turn harm the sealing effect. From this viewpoint, the sealing ring would in turn need to have a sufficiently large cross section to resist such deformations. Two conflicting requirements have thus to be met. In JP 2001-032935 A2, this is achieved by providing in addition to the sealing ring, radially on the inside, an expander ring, which presses the sealing ring, in addition to the working pressure, by a spring force mechanically against the piston ring.
An object of the present invention is therefore to at least reduce the above-stated drawbacks of known gastight piston ring arrangements.